A Geological Mystery

This weekend, I was upstate in the Hudson Valley visiting my parents, and we went hiking along the ridge of Schunemunk Mountain. While we were walking, I spotted the rock depicted in the image accompanying this post, a reddish pebble about a thumb length in diameter and cemented into a loose boulder of conglomerate the size of my head. You can probably see what it is about it that caught my attention: its surface was clearly marked with three dark, smooth, concentric ovals, almost perfectly regular, the same shape as the pebble itself. Whatever the markings are, they’ve been there long enough for lichen to partially grow over them.

I spent a while studying it, trying to figure out what might have created them. They look regular enough to be a natural formation, except I can’t imagine what kind of natural process would produce such perfect concentric shapes. (Ripple marks?) My second guess was a fossil, but they look too simple to be the trace of a living organism. (Some kind of print or burrow mark?) I also considered whether it could be a human-created marking, possibly by Native Americans who once lived in the area – but I don’t know if even they would have been around long ago enough for lichen to grow over their handiwork like this, and since there were no other markings on the rock that I saw, I couldn’t guess what the point would have been.

I looked up a webpage on the geology of the region, but it didn’t offer any clues. I’m not enough of a geologist to render a verdict, but maybe there’s someone reading this blog who is. So, I leave it up to you: Natural formation? Fossil? Petroglyph? Something else entirely? What do you think and why?